Black has just made a "beginner" move f7-f6 to protect the pawn on e5. White has many good choices here, because f7-f6 is a positionally terrible move. It blocks the f6 square for the knight on g8 and it weakens the light squares around the black king (f7, g6 and e6). White can play d2-d4 or Bf1-c4, they both look good. White can also try a direct refutation by playing Nf3xe5. But things are not very clear after Nxe5, because the position becomes more dynamic and the positional disadvantages created because of f7-f6 are replaced by dynamic factors. Of course Nxe5 would win directly if black played "normal" moves. But black could try to defend in creative ways and then it is up to white to discover how difficult Nxe5 can be!